Visual Short-Term Memory Operates More Efficiently on Boundary Features Than on Surface Features

Alvarez, George A., Cavanagh, Patrick, Perception and Psychophysics

A change detection task was used to estimate the visual short-term memory storage capacity for either the orientation or the size of objects. On each trial, several objects were briefly presented, followed by a blank interval and then by a second display of objects that either was identical to the first display or had a single object that was different (the object changed either orientation or size, in separate experiments). The task was to indicate whether the two displays were the same or different, and the number of objects remembered was estimated from the percent correct on this task. Storage capacity for a feature was nearly twice as large when that feature was defined by the object boundary, rather than by the surface texture of the object. This dramatic difference in storage capacity suggests that a particular feature (e.g., right tilted or small) is not stored in memory with an invariant abstract code. Instead, there appear to be different codes for the boundary and surface features of objects, and memory operates on boundary features more efficiently than it operates on surface features.

The short-term storage of visual information is a critical component of visual information processing. It enables incoming stimuli to be tracked and compared with current percepts and with other information already in short-term storage or in long-term storage. Early work on visual memory by Phillips (1974) revealed that there are two separate storage systems for visual information: One is a high-capacity sensory store, and the other is a shortterm store with a relatively limited capacity, referred to here as visual short-term memory. Recent work has shown that the upper limit on the number of objects that can be stored in visual short-term memory is quite small, on the order of about four or five simple objects (Luck & Vogel, 1997). The number that can be stored is limited both by an upper limit on the number of objects that can be stored and by the total amount of visual information that can be stored in memory. Thus, increasing the amount of information stored per object reduces the number of objects that can be stored (Alvarez & Cavanagh, 2004).

The question addressed in the present article is whether the same abstract code, such as right-tilted or small, is stored regardless of the format in which that information was initially presented. It is easy to imagine abstract codes for verbal memory. For example, if asked to store the uppercase letters A, P, Q, T in verbal memory, there would be no difficulty in recognizing the lowercase letters a, p, q, t as the same, despite the change in physical appearance, presumably because the information has been encoded in an abstract form. Similarly, work on visual memory suggests that letter shape can be encoded with a somewhat abstract structural description, so that memory for conjunctions of color and shape is insensitive to font changes that preserve letter structure (e.g., A vs. A), whereas large impairments occur with changes that alter letter structure (e.g., A vs. a; Walker & Hinkley, 2003). Thus, it appears that, in some cases, visual memory stores complex shape information with an abstract structural code.

In the present study, we explored whether boundary and surface features of objects are treated equally by visual short-term memory, as would be expected if visual short-term memory operates over abstract codes. Figure 1 illustrates the difference between boundary and surface orientation and between boundary and surface size. In Figure 1A two different objects with identical orientations are presented. On the left is a Gabor patch (a sine wave grating with contrast vignetted by a Gaussian envelope), and on the right is a single bar of the Gabor patch. Figure IB highlights the boundary of each object. The boundary of the Gabor has no clear orientation, whereas the boundary of the bar has a strong rightward tilt. In contrast, within the boundary, the Gabor surface texture has a clear rightward tilt, whereas the bar texture (homogeneous black) has no orientation information. …

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